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North Atlantic Barotropic Vorticity Balances in Numerical models

JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第1期46卷 289-303页

作者： Schoonover, Joseph Dewar, William Wienders, Nicolas Gula, Jonathan McWilliams, James C. Molemaker, M. Jeroen Bates, Susan C. Danabasoglu, Gokhan Yeager, Stephen Florida State Univ Inst Geophys Fluid Dynam 77 Chieftan Way Tallahassee FL 32306 USA Florida State Univ Dept Earth Ocean & Atmospher Sci Tallahassee FL 32306 USA Univ Calif Los Angeles Dept Atmospher & Ocean Sci Los Angeles CA USA Natl Ctr Atmospher Res POB 3000 Boulder CO 80307 USA

Numerical simulations are conducted across model platforms and resolutions with a focus on the North Atlantic. Barotropic vorticity diagnostics confirm that the subtropical gyre is characterized by an inviscid balance primarily between the applied wind stress curl and bottom pressure torque. In an area-integrated budget over the Gulf Stream, the northward return flow is balanced by bottom pressure torque. These integrated budgets are shown to be consistent across model platforms and resolution, suggesting that these balances are robust. Two of the simulations, at 100- and 10-km resolutions, produce a more northerly separating Gulf Stream but obtain the correct integrated vorticity balances. In these simulations, viscous torque is nonnegligible on smaller scales, indicating that the separation is linked to the details of the local dynamics. These results are shown to be consistent with a scale analysis argument that suggests that the biharmonic viscous torque in particular is upsetting the inviscid balance in simulations with a more northerly separation. In addition to providing evidence for locally controlled inviscid separation, these results provide motivation to revisit the formulation of subgrid-scale parameterizations in general circulation models.

关键词： Circulation Dynamics Baroclinic flows Barotropic flows models and modeling Coupled models General circulation models Model comparison Ocean models

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An Assessment of the Seasonal Salinity Budget for the Upper Bay of Bengal

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第5期46卷 1361-1376页

作者： Wilson, Earle A. Riser, Stephen C. Univ Washington Sch Oceanog Box 357940 Seattle WA 98195 USA

During each summer monsoon, the Bay of Bengal is inundated by a large amount of rain and river discharge. The effects of this freshening are gradually reversed over the course of the year, with near-surface salinities typically returning to their presummer monsoon levels before the start of the next rainy season. While the forcing responsible for the summertime freshening is clear, the processes that act to restore the bay's salinity are not well understood. To examine these processes, the authors construct a basin-integrated, near-surface, seasonal salinity budget using data-assimilated output from the Hybrid Coordinate Ocean Model (HYCOM). From this salinity budget, it is deduced that vertical salt fluxes are primarily responsible for counterbalancing the near-surface freshening caused by the summertime freshwater fluxes. These vertical salt fluxes are largest during the months that immediately follow the summer monsoon, when the near-surface halocline is strongest. These results must be tempered with the knowledge that HYCOM misrepresents some key features of the bay's salinity field. In particular, the model tends to overestimate salinity along the East Indian Coastal Current during its equatorward phase. Notwithstanding these biases, these results still suggest that vertical processes have a prominent role in the bay's near-surface salinity budget.

关键词： Geographic location/entity Indian Ocean Circulation/ Dynamics Hydrologic cycle Ocean circulation models and modeling Model evaluation/performance Variability Oceanic variability Seasonal variability

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Assessing the Surface Solar Radiation Budget in the WRF Model: A Spatiotemporal Analysis of the Bias and Its Causes

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MONTHLY WEATHER REVIEW 2016年第2期144卷 703-711页

作者： Ruiz-Arias, Jose A. Arbizu-Barrena, Clara Santos-Alamillos, Francisco J. Tovar-Pescador, Joaquin Pozo-Vazquez, David Univ Malaga Dept Appl Phys 1 Atmosphere & Solar Radiat Modeling Grp E-29071 Malaga Spain Univ Jaen Dept Phys Atmosphere & Solar Radiat Modeling Grp Jaen Spain

Solar radiation plays a key role in the atmospheric system but its distribution throughout the atmosphere and at the surface is still very uncertain in atmospheric models, and further assessment is required. In this study, the shortwave downward total solar radiation flux (SWD) predicted by the Weather Research and Forecasting (WRF) Model at the surface is validated over Spain for a 10-yr period based on observations of a network of 52 radiometric stations. In addition to the traditional pointwise validation of modeled data, an original spatially continuous evaluation of the SWD bias is also conducted using a principal component analysis. Overall, WRF overestimates the mean observed SWD by 28.9 W m(-2), while the bias of ERA-Interim, which provides initial and boundary conditions to WRF, is only 15.0 W m(-2). An important part of the WRF SWD bias seems to be related to a very low cumulus cloud amount in the model and, possibly, a misrepresentation of the radiative impact of this type of cloud.

关键词： Physical Meteorology and Climatology Cloud radiative effects Radiative fluxes Surface fluxes models and modeling Convective parameterization Cumulus clouds Numerical weather prediction forecasting

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A Simple Shelf Circulation Model: Intrusion of Atlantic Water on the West Spitsbergen Shelf

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第4期46卷 1209-1230页

作者： Nilsen, Frank Skogseth, Ragnheid Vaardal-Lunde, Juni Inall, Mark Univ Ctr Svalbard Longyearbyen Norway Scottish Assoc Marine Sci Oban Argyll Scotland

Barotropic flow along depth contours is found in accordance with standard geostrophic theory. A numerical model is developed that studies the deviation from such a flow. The model gives a good approximation of the dynamical processes on the West Spitsbergen Shelf (WSS) and shows that the West Spitsbergen Current (WSC), the main gateway of Atlantic water (AW) toward the Arctic, connects more easily to the Isfjorden Trough than anywhere else along the shelf. The circulation of AW in the troughs along the WSS is here named the Spitsbergen Trough Current (STC). From hydrographical and ocean current observations it is evident that the STC is primarily barotropic and driven by the sea surface height. A connection between the along-coast wind stress and the STC is established, and it is demonstrated how the increased occurrence of winter cyclones in Fram Strait during January-February accelerates and widens the WSC. Ultimately, this results in a strengthened STC and dominance of AW on the WSS. The STC represents a slower route of AW toward the Arctic Ocean and a large heat transport toward the West Spitsbergen fjords during winter (0.2-0.4 TW toward Isfjorden). Heat flux estimates show that half of the AW heat loss in the Isfjorden Trough is due to heat loss to the surrounding water masses, while the rest is lost to the atmosphere. Sea ice production along West Spitsbergen has been reduced, or even nonexistent, in some fjords since 2006. Here, the authors argue that this is a consequence of the strong southerly wind periods along the WSS during winter.

关键词： Upwelling/downwelling Barotropic flows Geographic location/entity Atmosphere-ocean interaction models and modeling Circulation/ Dynamics Topographic effects Quasigeostrophic models Arctic

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Reduction of Climate Sensitivity to Solar Forcing due to Stratospheric Ozone Feedback

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JOURNAL OF CLIMATE 2016年第12期29卷 4651-4663页

作者： Chiodo, G. Polvani, L. M. Columbia Univ Dept Appl Phys & Appl Math New York NY USA Columbia Univ Dept Appl Phys & Appl Math Dept Earth & Environm Sci New York NY USA Columbia Univ Lamont Doherty Earth Observ New York NY USA

An accurate assessment of the role of solar variability is a key step toward a proper quantification of natural and anthropogenic climate change. To this end, climate models have been extensively used to quantify the solar contribution to climate variability. However, owing to the large computational cost, the bulk of modeling studies to date have been performed without interactive stratospheric photochemistry: the impact of this simplification on the modeled climate system response to solar forcing remains largely unknown. Here this impact is quantified by comparing the response of two model configurations, with and without interactive ozone chemistry. Using long integrations, robust surface temperature and precipitation responses to an idealized irradiance increase are obtained. Then, it is shown that the inclusion of interactive stratospheric chemistry significantly reduces the surface warming (by about one-third) and the accompanying precipitation response. This behavior is linked to photochemically induced stratospheric ozone changes, and their modulation of the surface solar radiation. The results herein suggest that neglecting stratospheric photochemistry leads to a sizable overestimate of the surface response to changes in solar irradiance. This has implications for simulations of the climate in the last millennium and geoengineering applications employing irradiance changes larger than those observed over the 11-yr sunspot cycle, where models often use simplified treatments of stratospheric ozone that are inconsistent with the imposed solar forcing.

关键词： Physical Meteorology and Climatology Chemistry, atmospheric Climate sensitivity Feedback models and modeling Climate models

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Simulating the Mutual Forcing of Anomalous High Southern Latitude Atmospheric Circulation by El Nino Flavors and the Southern Annular Mode

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JOURNAL OF CLIMATE 2016年第6期29卷 2291-2309页

作者： Wilson, Aaron B. Bromwich, David H. Hines, Keith M. Ohio State Univ Byrd Polar & Climate Res Ctr Polar Meteorol Grp Columbus OH 43210 USA Ohio State Univ Atmospher Sci Program Dept Geog Columbus OH 43210 USA Ohio State Univ Byrd Polar & Climate Res Ctr Polar Meteorol Grp Columbus OH 43210 USA

Numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM) are conducted based on tropical forcing of El Nino flavors. Though these events occur on a continuum, two general types are simulated based on sea surface temperature anomalies located in the central (CP) or eastern (EP) tropical Pacific. The goal is to assess whether CAM adequately represents the transient eddy dynamics associated with each of these El Nino flavors under different southern annular mode (SAM) regimes. CAM captures well the wide spatial and temporal variability associated with the SAM but only accurately simulates the impacts on atmospheric circulation in the high southern latitudes when the observed SAM phase is matched by the model. Composites of in-phase (El Nino-SAM2) and out-of-phase (El Nino-SAM1) events confirm a seasonal preference for in-phase (out of phase) events during December-February (DJF) [June-August (JJA)]. Modeled in-phase events for both EP (during DJF) and CP (during JJA) conditions support observations of anomalous equatorward momentum flux on the equatorward side of the eddy-driven jet, shifting this jet equatorward and consistent with the low phase of the SAM. Out-of-phase composites show that the El Nino-associated teleconnection to the high southern latitudes is strongly modulated by the SAM, as a strong eddy-driven jet is well maintained by high-latitude transient eddy convergence despite the tropical forcing. A regional perspective confirms that this interaction takes place primarily over the Pacific Ocean, with high-latitude circulation variability being a product of both tropical and high-latitude forcing.

关键词： Antarctic Oscillation Circulation/ Dynamics Eddies General circulation models models and modeling Momentum Teleconnections Tropical variability Variability

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Prediction and Predictability of High-Impact Western Pacific Landfalling Tropical Cyclone Vicente (2012) through Convection-Permitting Ensemble Assimilation of Doppler Radar Velocity

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MONTHLY WEATHER REVIEW 2016年第1期144卷 21-43页

作者： Zhu, Lei Wan, Qilin Shen, Xinyong Meng, Zhiyong Zhang, Fuqing Weng, Yonghui Sippel, Jason Gao, Yudong Zhang, Yunji Yue, Jian Peking Univ Sch Phys Dept Atmospher & Ocean Sci Lab Climate & Ocean Atmosphere Studies 209 Chengfu Rd Beijing 100871 Peoples R China Nanjing Univ Informat Sci & Technol Key Lab Meteorol Disaster Minist Educ Nanjing Jiangsu Peoples R China Guangzhou Inst Trop & Marine Meteorol Key Lab Reg Numer Weather Predict Guangzhou Guangdong Peoples R China Penn State Univ Dept Meteorol 503 Walker Bldg University Pk PA 16802 USA IM Syst Grp Rockville MD USA NOAA Environm Modeling Ctr College Pk MD USA

The current study explores the use of an ensemble Kalman filter (EnKF) based on the Weather Research and Forecasting (WRF) Model to continuously assimilate high-resolution Doppler radar data during the peak-intensity stage of Tropical Cyclone (TC) Vicente (2012) before landfall. The WRF-EnKF analyses and forecasts along with the ensembles initialized from the EnKF analyses at different times were used to examine the subsequent evolution, three-dimensional (3D) structure, predictability, and dynamics of the storm. Vicente was an intense western North Pacific tropical cyclone that made landfall around 2000 UTC 23 July 2012 near the Pearl River Delta region of Guangdong Province, China, with a peak 10-m wind speed around 44 m s(-1) along with considerable inland flooding after a rapid intensification process. With vortex- and dynamics-dependent background error covariance estimated by the short-term ensemble forecasts, it was found that the WRF-EnKF could efficiently assimilate the high temporal and spatial resolution 3D radar radial velocity to improve the depiction of the TC inner-core structure of Vicente, which in turn improved the forecasts of the track and intensity along with the associated heavy precipitation inland. The ensemble forecasts and sensitivity analyses were further used to explore the leading dynamics that controlled the prediction and predictability of track, intensity, and rainfall during and after its landfall. Results showed that TC Vicente's intensity and precipitation forecasts were largely dependent on the initial relationship between TC intensity and location and the initial steering flow.

关键词： Circulation Dynamics Hurricanes Observational techniques and algorithms Radars Radar observations Forecasting Ensembles Mesoscale forecasting models and modeling Data assimilation Mesoscale models

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Sensitivity of Simulated Sea Breezes to Initial Conditions in Complex Coastal Regions

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MONTHLY WEATHER REVIEW 2016年第4期144卷 1299-1320页

作者： Lombardo, Kelly Sinsky, Eric Jia, Yan Whitney, Michael M. Edson, James Univ Connecticut Dept Marine Sci 1080 Shennecossett Rd Groton CT 06340 USA

Mesoscale simulations of sea breezes are sensitive to the analysis product used to initialize the simulations, primarily due to the representation of the coastline and the coastal sea surface temperatures (SSTs) in the analyses. The use of spatially coarse initial conditions, relative to the horizontal resolution of the mesoscale model grid, can introduce errors in the representation of coastal SSTs, in part due to the incorrect designation of the land surface. As a result, portions of the coastal ocean are initialized with land surface temperature values and vice versa. The diurnal variation of the sea surface is typically smaller than over land on meso- and synoptic-scale time scales. Therefore, it is common practice to retain a temporally static SST in numerical simulations, causing initial SST errors to persist through the duration of the simulation. These SST errors influence horizontal coastal temperature and humidity gradients and thereby the development of the sea-breeze circulations. The authors developed a technique to modify the initial surface conditions created from a reanalysis product [North American Regional Reanalysis (NARR)] for simulations of two sea-breeze events over the New England coast to more accurately represent the finescale structure of the coastline and the spatial representation of the coastal land surface and SST. Using this technique, the coastal SST (2-m temperature) RMSE is reduced from as much as 25 degrees-1 degrees C (7 degrees-1 degrees C), contributing to a more accurate propagation of the sea-breeze front. Techniques described in this work may be important for mesoscale simulations and forecasts of other coastal phenomena.

关键词： Mesoscale processes Forecasting models and modeling Numerical weather prediction/forecasting Sea breezes Physical Meteorology and Climatology Mesoscale forecasting Atm/Ocean Structure/ Phenomena

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Modifications to the Near-Storm Environment Induced by Simulated Supercell Thunderstorms

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MONTHLY WEATHER REVIEW 2016年第1期144卷 273-293页

作者： Nowotarski, Christopher J. Markowski, Paul M. Texas A&M Univ Dept Atmospher Sci 3150 TAMU College Stn TX 77843 USA Penn State Univ Dept Meteorol 503 Walker Bldg University Pk PA 16802 USA

This study investigates the changes that simulated supercell thunderstorms impart on their surroundings. Supercells are simulated in a strongly sheared convective boundary layer comprising horizontal roll vortices. In sensitivity tests, the effects of cloud shading on the near-storm environment are explored through the removal of cloud ice, water, and hydrometeor effects on parameterized radiation. All of the simulated supercells increase the low-level shear in their proximal environment;however, this effect is more pronounced when cloud shading is included. Shading stabilizes the boundary layer beneath the cirrus anvil, diminishes boundary layer rolls and their attendant thermodynamic perturbations, and reduces the intensity of resolved turbulent mixing in the convective boundary layer. Anvil shading also acts to reduce the buoyancy of inflow air and the horizontal buoyancy gradient along the forward-flank outflow boundary.

关键词： Circulation Dynamics Cloud forcing Convective storms Convective-scale processes Atm Ocean Structure Phenomena Severe storms Supercells models and modeling Nonhydrostatic models

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A PDF-Based Formulation of Microphysical Variability in Cumulus Congestus Clouds

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第1期73卷 167-184页

作者： Kogan, Yefim L. Mechem, David B. Univ Oklahoma Cooperat Inst Mesoscale Meteorol Studies Norman OK 73072 USA Univ Kansas Dept Geog Atmospher Sci Program Lawrence KS 66045 USA

Calculating unbiased microphysical process rates over mesoscale model grid volumes necessitates knowledge of the subgrid-scale (SGS) distribution of variables, typically represented as probability distribution functions (PDFs) of the prognostic variables. In the 2014 Journal of the Atmospheric Sciences paper by Kogan and Mechem, they employed large-eddy simulation of Rain in Cumulus over the Ocean (RICO) trade cumulus to develop PDFs and joint PDFs of cloud water, rainwater, and droplet concentration. In this paper, the approach of Kogan and Mechem is extended to deeper, precipitating cumulus congestus clouds as represented by a simulation based on conditions from the TOGA COARE field campaign. The fidelity of various PDF approximations was assessed by evaluating errors in estimating autoconversion and accretion rates. The dependence of the PDF shape on grid-mean variables is much stronger in congestus clouds than in shallow cumulus. The PDFs obtained from the TOGA COARE simulations for the calculation of accretion rates may be applied to both shallow and congestus cumulus clouds. However, applying the TOGA COARE PDFs to calculate autoconversion rates introduces unacceptably large errors in shallow cumulus clouds, thus precluding the use of a universal PDF formulation for both cloud types.

关键词： Physical Meteorology and Climatology Cloud microphysics models and modeling Cloud parameterizations Subgrid-scale processes

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